Electrical power generation and distribution, systems utilize a network of wires, connectors, transformers, and switches which are collectively known as switchgear. Reliability and efficiency of such systems are important concerns to operators—an unplanned service outage often carries with it a significant financial burden.
Energy loss in switchgear and conductors is proportional to the square of the current passing therethrough. Utilizing higher voltages causes a reduction of the current for transferring a given amount of energy. Therefore such systems utilize very high voltage ranging between several hundreds and well over a million volts. However, even at such high voltages, as the amount of energy is high, the system uses high current as well.
Generation and distribution systems are designed to have low resistance in the conductors; however aging infrastructure—especially at cable junctions and switch contacts—lead to increasing local resistance over time. The resistance increases cause local heating, which accelerates the aging of the junction or contact.
Oftentimes imminent failure of a connector or a switchgear element is generally accompanied by such elevated electrical resistance. When the electrical current transferred through the network flows through this resistance, it causes heat. Therefore, an imminent failure of a switchgear component may be predicted by elevated temperature. Other parameters such as vibration, humidity, and the like, also provide valuable data and it is often desired to perform such measurements in proximity to the relevant equipment.
Placing measurement devices in close proximity to high voltage lines presents a slew of technical problems. While temperature, humidity, acceleration and other sensors are well known, the better ones operate at low voltage, and are sensitive to electrostatic discharge. If wired sensors are used, isolating the sensors and wires from the high voltage fields in which they are required to operate is difficult, and utilities and industrial users are often unwilling to accept the associated risks even when such use is possible.
Wireless measuring devices which receive their energy from an external transmitter are well known. The measuring device generally comprises a sensor, and at least one antenna and in certain cases the sensor comprises optional circuitry for coupling between the sensing element(s) and the antenna. When exposed to a pre-set frequency signal from the transmitter, such devices re-radiate radio frequency (RF) energy, or otherwise disrupts the RF energy field, in a manner that conveys information about the parameter they are designed to measure. The re-radiated energy or field disruption is received by a receiver which allows the information to be used. This technology is colloquially known as ‘passive device’. Notably, in certain embodiments the antenna may be directly coupled to the sensor and added circuitry is minimal or not required. Oftentimes, the measuring device is located on the arm of a switch or relay contact, at as small a distance as practical therefrom. In connectors, the sensor will often be in direct contact with the connector or one of the wires.
Several characteristics are desired from an antenna which is introduced to such high voltage electrical fields. Firstly, it is desirable that the antenna will have low induced voltage in the frequency of the high voltage fields, commonly known as the line frequency. This prevents exposure of the sensor to high voltages, and to varying voltages according to the instant current in the device, which may vary widely. Most power transmission in today's equipment occurs between the DC—zero frequency range—and line frequency of 400 Hz, thus it is desirable to limit the antenna induction in those frequencies. Secondly, minimizing the size of the antenna is highly desired as doing so will require minimal changes to existing switchgear design, and will very often allow for retrofitting of the sensor to existing design.
Many typical devices, most notably surface acoustic wave (SAW) resonators and delay lines, have very low immunity to electrostatic discharge (ESD) or to applied electric fields in excess of the millivolt and smaller radio frequency signals for which they are designed. Directly coupling an antenna to such devices may induce hundreds or thousands of volts of electrostatic potential at the line frequency. This electrostatic potential may be reduced by a resistive or inductive shunt protective element across the SAW device; however the shunt protective element invariably reduces the radio frequency efficiency of the sensor and decreases the distance and/or signal to noise ratio at which it can be monitored. It is desirable to employ antenna structures with little or no induced electrical potential from the power system so as to eliminate or minimize the ESD protection required.
It is also important that the antenna will present a minimum source of corona discharge. Corona discharges are disruptive as they damage insulation, create ozone and in extreme cases may cause arcing sparks, and shortage. Generally, the larger the radius of curvature and the larger the separation between conductors at different potentials, the larger a potential difference is required to initiate a corona.
Monopole antennas present a small and efficient antenna for the measuring devices. However monopole antennas generally require a ground. When dealing with radio frequency, the term ‘ground’ may refer to “RF ground” rather than to actual ground. RF ground refers to the potential neutral relative to an RF voltage or wave. While earth ground is often used as RF ground, such use is not a necessity. RF ground has no relationship with earth ground and a cable or contact of sufficient size might be energized to hundreds of thousands of volts relative to earth ground and still be considered RF ground. The term ‘ground plane’ refers to the plane from which ground reflections may be considered to take place, i.e. the term relates to an electrical counterpoise or object which acts similar to a grounded object for purposes of radiation relative to the antenna.
A straight monopole antenna is often impractical within the confines of a switchgear device, especially when located within the confine of a spark arrestor, or the like. Helical monopole antennas are known, where the required electrical length—an odd integer multiple of quarter wavelengths (λ/4)—is formed into a coil. FIG. 1 represents an example of such conventional design. A length of wire is wound to a coil 10, and is connected on one side to the measurement device 20, while the other side terminates in a high impedance at a point 40. A relatively large metal element 30 acts as a ground plane and point 40 is capacitively coupled to said ground plane as is the length of the coil itself through parasitic capacitance of the surrounding medium, typically air. The skilled in the art would recognize that the ground plane adds significant bulk to the device, and that the point 40 acts as a point electrode which increases the potential for corona discharge between the antenna distal end and surrounding objects.
There is therefore, a long and heretofore unfulfilled need in the art, for an efficient antenna design which will occupy a relatively small volume, without utilizing corona discharge prone point electrodes, and generally protected from corona discharge.